The present invention relates generally to a method for fabricating a semiconductor device, and more particularly, to a method for forming an interlayer dielectric for isolating bit lines from each other.
As the integration level of a semiconductor device increases and the size of devices is reduced, a gap-fill of a dielectric layer for isolating the devices or conductive layers becomes more difficult. Polysilazane (PZT) used for a gap-fill of a minute region has been recently introduced. The PZT is also called silicon on dielectric (SOD), and has a good reflow property to thus easily gap-fill a minute region.
Unlike a high-density plasma (HDP) oxide layer deposited using chemical vapor deposition (CVD), the SOD layer is coated using coating equipment and then cured at a predetermined temperature using curing equipment so that silicon is transformed to silicon oxide (SiO2). That is, the SOD layer is coated, and then heat treated in a vapor atmosphere to replace silicon (Si)-nitrogen (N)-hydrogen (H) atoms with silicon oxide (SiO2). In this process, properties of the silicon oxide (SiO2) layer vary with curing conditions. Since the property of the silicon oxide (SiO2) greatly affects a following wet etching process, it is very important to select an appropriate curing condition for forming a more stable silicon oxide (SiO2) layer.
In addition, the SOD layer is being used instead of the HDP oxide layer as an interlayer dielectric for isolating bit lines or isolating bit lines and other conductive layers in view of the gap-fill characteristic. In order to form the interlayer dielectric using the SOD layer, the SOD is coated to a predetermined thickness on a semiconductor substrate where bit lines are formed, and cured in a wet atmosphere using water vapor (H2O) or a dry atmosphere using oxygen (O2). If a furnace is used to cure the SOD layer, the curing is performed at a flow rate ratio of hydrogen (H2) gas to oxygen (O2) gas of 1.85 or less in order to prevent explosion due to non-reacted hydrogen (H2) gas remaining at a high temperature. Therefore, the curing of the SOD layer is performed in a relatively rich oxygen (O2) gas atmosphere.
However, unlike the case of filling trenches with the SOD layer to form a device isolation layer, in the gap-fill process after a bit line forming process, underlying bit lines are easily oxidized by oxygen (O2) during the curing of the SOD layer because the bit lines are formed of metal, such as tungsten (W), which is susceptible to oxidation. The oxidation of the bit lines increases the resistance of the bit lines, deteriorates electrical characteristic of a device, and thereby reducing the yield rate.